I. Field of the Invention
This invention relates generally to a method of forming particle board and the like, using an exterior grade resin adhesive, specifically a polyisocyanate and a phenolic, e.g., phenol/formaldehyde adhesive.
II. Description of the Prior Art
Phenolic resole resins prepared by the alkaline condensation of a phenol and formaldehyde such as disclosed in U.S. Pat. Nos. 2,360,376 to VanEpps, 2,437,981 to Stephan, and 2,457,493 to Redfern have long been used in plywood adhesive technology. Similar basic formulations and technology has been employed in formulation of an economical fast-cure phenolic resin for exterior hardwood flakeboard, such as described by Chung-Yun Hse in Formulation of an Economical First Cure Phenolic Resin for Exterior Hardwood Flakeboard, Proceedings of the Ninth Particleboard Symposium, Washington State University, Pullman, Washington, 1975.
It has also been shown that the wide variation in hardwood density and process variables, such as moisture content of wood chips or flakes significantly affects resin performance. Additionally, it has been noted that the wood absorbs water from the adhesive glue during the mat forming process. Thus, with highly absorbent hardwood species such as, for example, the oaks, rapid dehydration of the glue line occurs with resultant inadequate glue flow and penetration during the subsequent hot press cure cycle. With a less absorptive species, such as, for example, sweetgum, an aqueous glue line dries more slowly, thereby causing unacceptable flow and penetration during hot pressing. Either of these latter conditions causes a substandard flakeboard.
Advantageously, a good hardwood flakeboard adhesive must not only be capable of producing specification-grade flakeboard over a broad range of wood species and density, but must also be tolerant of the variable wood absorptivity. The adhesive should also be characterized by tolerance of high flake moisture content and temperature, high temperature and humidity of the working area, low resin application, and press pre-cure time, etc.
Various adhesives and adhesive compositions have been previously proposed for use in fabricating flake board, particle board and the like, for example U.S. Pat. No. 3,919,017 to Phillip D. Shoemaker et al discloses the bonding of cellulosic materials to form particle board and the like with a binder system comprising an organic polyisocyanate and formaldehyde. The use of this adhesive system is said to allow the cellulosic materials containing up to about 22% moisture to be pressure consolidated without predrying. With regard to this combined adhesive of a polyisocyanate and formaldehyde, the patentees suggest that the binder system components can be separately added, although the order of addition is unimportant and the components may be added simultaneously. The patentees further indicate that typical adhesive systems for fabrication of particle board have included such thermosetting resins as phenolformaldehyde resorcinol-formaldehyde, melamine-formaldehyde and others.
U.S. Pat. No. 3,440,189 to John Arthur Sharp discloses the fabrication of particle board using as the adhesive a thermoset adhesive containing an organic polyisocyanate and "Vinsol" resin. The "Vinsol" resin is described as a resinous hydroxyl group containing component derived from pine wood. The use of this combination adhesive is said to produce a water resistant particle board suitable for outdoor applications.
Other U.S. Pat. Nos. which show typical adhesive compositions for the fabrication of particle board and the like include: 3,821,056 to Joseph Edward Wierden-polyisocyante/polyol reaction product; 3,632,734 to John G. Haygreen-application of phenol/formaldehyde resin to green particles; 3,736,275 to Nicholas J. Iamnarino-Xanthomonas hydrophilic colloid and cross linking agent; 3,874,990 to Lyle V. Surdyk-flame retardant phosphoric acid-dicyandiamide formaldehyde resin; and 3,968,308 to Aldoph Buschfeld et al-a phenol or urea condensation resin.
Typical processes for fabricating particle board and the like are shown in the foregoing patents as well as the following: U.S. Pat. Nos. 3,492,388 to J. England-Kanusel; 3,557,263 to Allen A. Marra; 3,649,396 to B. J. Carlsson and 3,899,559 to Fred E. Johnanson et al.
In an environment unrelated to the fabrication of particle board and the like, the combination of a polyisocyanate and a formaldehyde condensate has been proposed. Thus U.S. Pat. No. 3,268,467 to Grover W. Rye et al proposes the blocking of a polyisocyanate with a formaldehyde condensate resin in bonding a rubber reinforcing element to rubber, such as for example in bonding rayon, nylon, dacron or a metal such as steel to rubber in the tire industry.